A ratio buster is an online player, in this case a player on Call of Duty 4. Not always highly skilled, more often than not, lucky. He/she will repetedly kill you despite ones best efforts to evade or extermanate him/her. Resulting in a drop in your kill/death ratio.
Me: "Yo G! Check that guy ratio busting up on the roof."
G: "Tell me about it. That motherfucker has just put me down. Fuckin Ratio Buster!"